It is known that optical fibers having a doped core obtained by the use of particular substances, such as rare earth ions, have stimulated emission features adapted for use as laser sources and optical amplifiers.
In fact, these fibers can be supplied with a light source of a particular wavelength, referred to as pumping wavelength, which is capable of bringing the dopant atoms to an excited energy state, or pumping band, from which the atoms spontaneously decay within a very short period of time into a laser emission state, in which state they remain for a relatively long period of time.
When a fiber having a high number of atoms at the excited state in the laser emission level is transited by a light signal having a wavelength corresponding to that laser emission state, the signal causes the transition of the excited atoms to a lower level producing light emission having the same wavelength as the signals. Therefore, a fiber of the such kind can be used to achieve an amplification of the signal and in particular, for example, to achieve optical line amplifiers adapted to bring an attenuated transmission optical signal back to a high level after a long travel through a fiber in a telecommunication line.
Optical amplifiers of the such kind are, for example, known from the U.S. Pat. Application Ser. No. 602,364 filed Oct. 22, 1990 and assigned to the assignee of this application, in which the active fiber is of the single-mode type both at the transmission wavelength and at the pumping wavelength.
However, such fibers, which are single-mode both at the transmission wavelength and pumping wavelength, have a different distribution of the luminous power in the fiber section, and in particular, the luminous power of the transmission signal is distributed over an area of the fiber section which is greater than the area in which the pumping power is present.
The fluorescent dopant, responsible for the transmission signal amplification, is concentrated in the fiber core and the fiber in known amplifiers is designed so that the pumping power is also confined in said area and, so that it can be entirely used to excite the fluorescent dopant in the laser emission level. However, since part of the transmission signal power is transmitted to the fiber outside the area in which the fluorescent dopant and pumping power are present, the result is that only part of said signal is available in the fiber area in which it can be amplified.
The above phenomenon brings about a limitation in the amplification efficiency of the amplifier by which is meant the gain of the transmission signal per pumping power unit.
Consequently, the problem arises of increasing said efficiency with respect to known amplifiers.